I went out and shot a local fire hydrant myself with the SD-1, and DP-3m... I was able to see the same thing, the DP-3M was red as it should be, the SD-1 hydrant was oddly orange. Both shots used Auto white balance and neutral color mode.

The solution in my tests ended up being to use Standard, not Neutral, color mode for that image, along with setting the color adjustment dropper on something neutral (in my case a street) in the SD-1 image (I did the same thing in the DP-3M image but it had no effect). Then the hydrant was red and the other colors matched also. You don't have to re-shoot, just change in SPP.

I normally advocate using Neutral all the time, and in my shooting I mostly found it to be the most accurate. I don't know what it is on the SD-1 that causes hydrant red to come out so orange in Neutral. Perhaps over some firmware updates Standard on the SD-1 has drifted closer to being accurate than Neutral...

The hydrant in the SD-1 image was also a touch more vibrant at that point, I had to bring saturation on the SD-1 image down about -0.1 for a closer match (it was a little under then).

I also took a picture of a construction-orange / blue sign as well, and found the same thing - to have the SD-1 more closely match the DP-3M image in Neutral mode, I switched the SD-1 image to Standard color mode.

Hi, I have a new SD1M that produce colors not true: some reds are orange, some greens are light blue; I've tried to change wb and to set up a personal wb, but with no result...which could be the cause?

This seems like a similar situation to the green cast issue of the SD14. With that we saw unit variations, but a camera dependent correction value in SPP fixed the issue. I recall Guenther Borgermeister reported he had to use different corrections for his 3 (?) cameras. And saw none of the issue in an other users SD14.

With this SD1 thing, Kendall verifies that the issue is real. Tom pointed to a post by Cinefeel, with a suggested correction value: -15Y +5R. (Might not be SPP, so a value has to be found there)

SO anfat, try some correction around what Cinefeel suggest that and see if you get closer. A workaround, yes, but not a big one.

That orangy red is an old friend for me - I once sold a perfectly good SD9 because of it. The same cast occasionally shows up on my SD10, no idea why. Artificial lighting did it on the SD9, even under tungsten with custom WB set. I tried SPP 2, 3, 4 and 5 . . nada. CFL's, tubes, candles, halogens, nuclear bombs, you name it, no difference until I went outside and kapow! - red reds suddenly appeared, so I do understand the frustration in the OP.

I shot the card at ISO 200, bracketing from -3 to +3 in 1EV steps, here is a screen capture:

The shots above 0 were blown out and were discarded. The white balance was set from the gray patch on the card in ACR 5.4 and opened in PSE6 from which the screen capture was done. Each exposure was adjusted in ACR to make the histograms similar. The under-exposed red at the top looks quite familiar. That red has no blue in it, so something got clipped somehow. RGB equaled 195, 70, 0. Hue and Sat were 22 deg, 100%, obviously wrong.

Perhaps the OP should measure his orange-colored reds' RGB values (not with a screen picker) and deduce something from that?

the same picture with the sd1m and with dp2m...that one shot with dp2m has true colors...

That really looks similar to the case I had. If you change the color mode to "Standard" and click the white balance dropper on the black gravel, the slide should go red... you'll notice that in the image as you have it from the SD-1, the rocks are very blue compared to the more neutral DP-2M image.

The only reds I have from the loaner SD1 are shots of red oriental rugs, which are fine, true tones (not online) and a couple of me in my red coat in Death Valley, taken by my husband. Coat color is exactly right looking at the RAWs (not online).

Red becoming orange or pink is an old Foveon problem. It has been seen for SD14 and at least for the earlier DP cameras. And obviously some here have seen it with their SD1M cameras. Strange that they have not told it already. If its common, it might be an important information for potential buyers.

As far as I understand, you always get it. Which, of course, not is acceptable. So ... you have to do something. If you cant find any solution ... I assume you have to contact Sigma about it.